Thermoplastic polymers are widely employed to create a variety of products, including blown and cascade films, extruded sheets, foams, fibers and products made therefrom, woven and knitted fabrics, and non-woven fibrous webs. Many thermoplastic polymers used in these products, such as polypropylene, are inherently hydrophobic, and there are a number of uses for thermoplastic polymers where their hydrophobic nature either limits their use or requires some effort to modify the surface of the shaped articles made therefrom. For example, polyolefins are used in the manufacture of nonwoven webs (that are employed in the construction of absorbent articles such as diapers, feminine care products, and personal incontinence products) and the use of such nonwoven webs are limited because of their hydrophobic nature. Polyolefins are particularly desirable for these applications because they are lower cost and provide a soft “hand” (e.g., a soft feel to the nonwoven web) due to the relatively low glass transition temperature.
Coating methods to provide a hydrophilic surface are known, but also have some limitations. The extra step required in coating preparation is expensive, time consuming and requires specialized equipment. Many of the solvents used for coating are flammable liquids or have exposure limits that require special production facilities. When coated out of aqueous solutions, the water must be subsequently removed in an oven, which is energy intensive and costly since water has a high heat of vaporization, causing line speeds to be slow. Furthermore, the quantity of any surfactant used to provide the hydrophilic surface is limited by the solubility in the coating solvent, the amount of the coating applied, and potential foaming problems.
WO 92/18569 and WO 95/01396 (Sargent et al.) describe fluorochemical additives for use in the extrusion of thermoplastic polymers to prepare films and fibers with anti-wetting (repellent) and antistatic properties.
The addition of one or more surfactants to the melts of thermoplastic polymers to impart hydrophilicity to both the surface and the bulk of the fiber is also known. U.S. Pat. Nos. 4,857,251 and 4,920,168 (Nohr et al.) describe a method of forming fibers by melt-extrusion of a surface-segregatable thermoplastic composition that comprises thermoplastic polymer and a siloxane-containing additive having certain moieties.
Fluorochemicals and/or fluoro-containing groups are also known to impart hydrophilicity to fibers. See, e.g., U.S. Pat. No. 5,804,625 (Temperante et al.); EP Patent No. 0 516271 (Gardiner); U.S. Pat. Nos. 5,244,951 and 5,300,357 (Gardiner) and U.S. Pat. No. 7,230,043 (Klun).
Other additives have also been described. U.S. Pat. No. 4,189,420 (Sugimoto) discloses surface wetting agents selected from polyethylene alkyl ethers, polyoxyethylene fatty acid esters, sorbitan fatty acid esters, sodium dialkylsulfosuccinate, mono- and diglycerides, as well as polyglycerol fatty acid esters. U.S. Pat. No. 5,087,520 (Suzuki et al.) describes fibers comprising a polyolefin or polyester having a mixture of a fatty acid diethanolamide, a polyether-modified silicone, a sorbitan fatty acid ester and a metal salt of an alkylsulfonate.
Certain classes of hydrocarbon, silicone, and fluorochemical surfactants have each been described as useful for imparting hydrophilicity to polymers. These surfactants typically are contacted with the thermoplastic polymer in one of two ways: (1) by topical application, e.g., spraying or padding or foaming, of the surfactants from aqueous solution to the extruded nonwoven web or fiber followed by drying, or (2) by incorporation of the surfactant into the polyolefin melt prior to extrusion of the web. The latter is more preferable but it is difficult to find a surfactant that will reliably bloom to the surface of the fiber or film in sufficient amount to impart hydrophilicity, and then remain properly oriented at the surface to ensure durable hydrophilicity. As previously described, webs made hydrophilic by topical application of a surfactant suffer many drawbacks. Some web constructions are reported to also have diminished hydrophilicity after a single contact with aqueous media.
Disadvantages to topical application of a surfactant to impart hydrophilicity, such as described above, can include skin irritation from the surfactant itself, non-uniform surface and bulk hydrophilicity. Incorporating one or more surfactants into the thermoplastic polymer as a melt additive alleviates the problems associated with topical application and in addition may provide a softer “hand” to the fabric or nonwoven web into which it is incorporated.
There continues to be a need to obtain durable wettability of polyolefin (e.g. polypropylene) fibrous nonwovens using melt additive surfactants that are nontoxic and preserve or improve the “hand” of the fabric.